The present invention relates to providing a dielectric layer for polymer dispersed liquid crystal displays.
Currently, information is displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically writable areas to carry ticketing or financial information, however magnetically written data is not visible.
A structure is disclosed in PCT/WO 97/04398, entitled xe2x80x9cElectronic Book With Multiple Display Pagesxe2x80x9d which is a thorough recitation of the art of thin, electronically written display technologies. Disclosed is the assembling of multiple display sheets that are bound into a xe2x80x9cbookxe2x80x9d, each sheet is arranged to be individually addressed. The patent recites prior art in forming thin, electronically written pages, including flexible sheets, image modulating material formed from a bi-stable liquid crystal system, and thin metallic conductor lines on each page.
Fabrication of flexible, electronically written display sheets are disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent ITO conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be glass, but in practice have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential applied to opposing conductive areas operate on the liquid crystal material to expose display areas. The display uses nematic liquid crystal material which ceases to present an image when de-energized.
U.S. Pat. No. 5,223,959 discloses a plurality of polymer dispersed liquid crystal material, each having a different dye material of red, green or blue dye material. Differing electrical signals to common electrodes operate on each of the materials to control the state of each type of dyed liquid crystal material. The patent requires the use of conventional nematic liquid crystals with a dye to absorb light. The droplets are chemically treated to be stable in either a clear or a light absorbing state. The invention also requires materials having different response times to electrical signals. The device must be continually driven so that the human eye perceives complementary colors. This arrangement has the disadvantage of requiring continuous, high speed electrical drive because the materials do not maintain their state. The material must be driven to achieve a neutral color density.
U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a polymer dispersed chiral nematic liquid crystal. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal-conic state. Said structure has the capacity of maintaining one of the given states in the absence of an electric field.
U.S. Pat. No. 3,816,786 discloses droplets of cholesteric liquid crystal in a polymer matrix responsive to an electric field. The electrodes in the patent can be transparent or non-transparent and formed of various metals or graphite. It is disclosed that one electrode must be light absorbing and it is suggested that the light absorbing electrode be prepared from paints contains conductive material such as carbon.
U.S. Pat. No. 5,289,300 discusses forming a conductive layer over a liquid crystal coating to form a second conductor. The description of the preferred embodiment discloses Indium-Tin-Oxide (ITO) over a liquid crystal dispersion to create a transparent electrode.
Prior art discloses the use of dielectric barrier layers formed over ITO conductors. The dielectric layer protects the ITO transparent conductor from damage from electrochemical interaction with the light modulating material. The protective layers are typically formed by vacuum sputtering silicon dioxide over the ITO conductors. The vacuum forming process is slow and expensive.
It is an object of this invention to provide a highly effective dielectric coating over electrodes used in polymer dispersed liquid crystal displays.
This object is achieved in a method of making a liquid crystal display, comprising the steps of:
(a) providing a substrate;
(b) providing a first electrode over the substrate;
(c) coating the first electrode with aqueous dispersed material which when dried provides a dielectric layer over the first electrode;
(d) coating the dielectric layer with liquid crystal bearing material and drying such liquid crystal bearing material; and
(e) providing a second electrode in contact with the dried liquid crystal bearing material.
The invention provides an inexpensive dielectric layer between field carrying electrodes in displays that are aqueous coated. Such dielectric layers provide a good bond between the aqueous suspension and the ITO on a flexible substrate.
The present invention provides a dielectric layer over a conductive layer using simple, inexpensive aqueous coatings. Such coatings permit the fabrication of electronic privacy screens having long life and durability. The aqueous dielectric coating significantly improves yields of cholesteric memory displays having electrical electrodes applied over a polymer dispersed liquid crystal layer.